The Jurassic/Cretaceous (J/K) boundary, 145 Myr ago, remains as the less understood of major Mesozoic stratigraphic boundaries. Sedimentological, palynological and geochemical studies, indicate a climatic shift from predominantly arid to semi arid conditions in the latest Jurassic to more amicable humid conditions in the earliest Cretaceous. The continued fragmentation of Pangaea across the Late Jurassic and Early Cretaceous led to large-scale tectonic processes, on both regional and global scale, accompanied by some of the largest volcanic episodes in the history of the Earth; eustatic oscillations of the sea level; potentially heightened levels of anoxia, oceanic stagnation, and sulphur toxicity; along with two purported oceanic anoxic events in the Valanginian and Hauterivian. There’s also evidence of three large bolide impacts in the latest Jurassic, one of which might have been bigger than the end-Cretaceous Chicxulub impact.

Painting of a late Jurassic Scene on one of the large island in the Lower Saxony basin in northern Germany (From Wikimedia Commons)

The J/K interval represents a period of elevated extinction, and involves the persistent loss of diverse lineages, and the origins of many major groups that survived until the present day (e.g. birds). The magnitude of this drop in diversity ranges from around 33% for ornithischians to 75–80% loss for theropods and pterosaurs. Mammals suffered an overall loss of diversity of 69%. Crocodyliforms suffered a major decline across the Jurassic/Cretaceous boundary in both the marine and terrestrial realms; while non-marine turtles declined by 33% of diversity through the J/K boundary. In contrast, lepidosauromorphs greatly increased in diversity (48%) across the J/K boundary, reflecting the diversification of major extant squamate clades, including Lacertoidea, Scincoidea and Iguania.

In the marine realm, sauropterygians and ichthyosaurs show evidence for a notable decline in diversity across the J/K boundary, which continued into the Hauterivian for both groups.

Eustatic sea level is the principal mechanism controlling the Jurassic–Cretaceous diversity of tetrapods. Rising sea levels leads to greater division of landmasses through creation of marine barriers, modifying the spatial distribution of near-shore habitats and affecting the species–area relationship, which can lead to elevated extinctions. This fragmentation can also be a potential driver of biological and reproductive isolation and allopatric speciation, the combination of which we would expect to see manifest in the diversity signal. Additionally, the diversity of fully marine taxa was more probably affected by the opening and closure of marine dispersal corridors, whereas that of terrestrial and coastal taxa was more probably dependent on the availability of habitable ecosystems, including the extent of continental shelf area (Tennant, et al., 2016).